Conventionally, waterproof air-permeable filters have been used, for example, for automobile electrical components, office automation apparatuses, household electrical appliances, and medical equipment in order to eliminate a pressure difference between the inside and outside of a housing accommodating electronic parts, control boards, etc. The waterproof air-permeable filter is attached to the housing so as to close an opening of the housing. The waterproof air-permeable filter serves to provide protection against dust and water while ensuring air permeability. In such a waterproof air-permeable filter, a PTFE porous membrane commonly is used.
Preferably, the PTFE porous membrane used in the waterproof air-permeable filter has a high water pressure resistance in order to ensure waterproofness, and has a high air permeability in order to eliminate the pressure difference.
Generally, the PTFE porous membrane is obtained by stretching an unsintered sheet made of PTFE. The air permeability of the PTFE porous membrane can be increased by increasing the stretching ratio, but the water pressure resistance is lowered accordingly. Thus, it is one of the difficult techniques in this field to produce a PTFE porous membrane having the air permeability and water pressure resistance in good balance.
In the waterproof air-permeable filter, it is general in many cases that the PTFE porous membrane is laminated with a support material, such as a nonwoven fabric, to be reinforced and the support material is welded to a housing. In such cases, the support material melted when being welded may damage the PTFE porous membrane. To deal with this, it is effective to increase the thickness of the PTFE porous membrane, but this decreases the air permeability. As just described, it is difficult to obtain a PTFE porous membrane with a large thickness as well as a high air permeability and water pressure resistance in the case of using a single-membrane structure.
For example, as disclosed in Patent Literature 1, a PTFE porous membrane with a laminated structure in which PTFE porous layers are laminated in order to prevent clogging is proposed as a PTFE porous membrane used as an air filter medium.